Make the automatic Kanban board refresh interval configurable

Currently, Kanban boards are configured with a hard coded 30 seconds refresh interval to check for board updates. If the board usage in the instance is high, and users tend to leave on for prolonged periods of time (e.g. as wallboards), boards will continuously poll Jira for updates, with each poll being a somewhat expensive operation on the Jira server resources.

For such environments, it would make sense to decrease the automatic polling interval to decrease the load on the server.

Workaround

None